For DPS gangplank, I always start with meki health and mana pot, or 2 health pots. Basically, I just try to lane enough for a BF sword + boots of speed, if I can't, I buy a zeal + boots of speed, I usually just lane until I get my IE or BF sword, and use my ultimate accordingly. After I get my IE, I get phage which turns to trinity force with the zeal, I just roam around after I get my trinity force and mercury treads, and basically, you don't ever really need to recall unless you really want to shop, which I usually farm enough for my Bloodthirster, but if I do need to recall, I just buy another zeal which turns to phantom dancer after my Bloodthirster. Last item, I always get a frozen heart, great cooldown reduction, gives mana, plenty of armor, and a good debuff.
As for tankplank, I don't play it much but, main items I get are atma's impaler, warmogs, sunfire cape, mercury treads and spirit visage.
As for AP build on gangplank, I don't see much point in this, so I don't play it.